1. Technical Field
The invention relates generally to energy-based systems, and in particular to a method and system for battery charge control based on a cycle life parameter.
2. Discussion of the Background Art
It is known to provide energy storage in the form of lithium chemistry batteries, as seen by reference to U.S. Pat. No. 6,063,519 to Barker et al. One characteristic of lithium chemistry batteries is that they have less tolerance to overcharging than the other battery technologies, such as nickel metal hydride (NiMH), lead acid (PbA) and nickel cadmium (NiCd) technologies. For example, heat may be generated due to overcharging, which may seriously impair or damage the battery. Another characteristic of lithium batteries is that the available capacity (e.g., watt-hours) varies as a function of a charge voltage. That is, current is passed through the battery when recharging, and the battery output voltage increases in response thereto. The charge voltage is the voltage at which charging is stopped. However, lithium batteries degrade with respect to capacity (e.g., watt-hours) with increasing charge voltages. This is due, in part, to the stress imposed on the cells, which increases with increasing voltage. While it is possible to charge lithium batteries to lower charge voltages, to ostensibly reduce stress, the downside is that the battery will have a reduced usable capacity.
Regarding charging regimens, it is known to provide a battery charger that provides the same charge to the battery every time. It is also known to provide a battery charger that adjusts a charging function based on changes in temperature and/or voltage, but such charging function remains constant through the life of the battery. It is also known to provide a charging regimen that inhibits both overcharging as well as deep discharging, as seen by reference to U.S. Pat. No. 5,839,816 to Varga et al. entitled ROAD MARKER. Varga et al disclose a battery-powered road marker, which includes a battery management feature for a NiCd battery such battery management feature inhibits charging when the battery output voltage reaches an upper limit to prevent overcharging and possible outgassing. Varga et al. further disclose another feature wherein all discharge is inhibited in the event that the battery system reaches a predetermined, lowest safe output voltage to prevent deep discharge of the battery. The battery management features disclosed in Varga et al. do not appear to change during the life of the battery system. However, some types of battery packs, generally, and lithium batteries in particular, change significantly with respect to capacity during its life cycle.